The present invention relates to a video signal compression apparatus for compressing component portions of a video signal according to compression ratios specific to each component which depend upon the frequency with which the particular component appears in the video signal.
During ordinary operation of a video camera system, the quantity of light incident upon the imaging device varies by several orders of magnitude. To facilitate the storage and reproduction of images comprised of such a range of light intensities, video signal compression techniques are employed to reduce the amount of information needed to represent such images.
In the art, an apparatus for compressing the intensity or luminance level of a video signal is referred to as a "knee circuit." Several examples of prior art knee circuits are described in U.S. Pat. No. 5,357,279, Nakamura et al., which is also assigned to the assignee of the present application. Typically, a knee circuit processes an input video signal according to the fixed transfer function illustrated in FIG. 1. According to this function, output video level varies with input video level at a first constant rate until the single "knee point" is reached. The "knee point" is simply the input video level at which the transfer function changes. After the knee point, output video level varies with input video level at a second constant rate, wherein the second constant rate is less than the first constant rate.
Upon study it has been determined that the frequency with which a particular input video level occurs is not necessarily proportional to the input video level. For example, as illustrated in the histogram of FIG. 2, the distribution of input video levels may be generally concentrated around two particular input video signal levels. In this figure, the y-axis represents the number of occurrences while the x-axis represents input video intensity level. The number of occurrences preferably refers to the number of imaging elements, e.g. pixels, which register a particular intensity or luminance level in a given input image.
Application of the fixed transfer function described above to the input image represented by FIG. 2 clearly yields an inefficient allocation of the output video signal. According to such a compression scheme, input video levels are compressed at one of two different rates regardless of the actual frequency with which a particular input video level occurs. Consequently, some input video levels are unnecessarily excessively compressed while other input video levels are allocated too much of the output video signal and thus are compressed by a less than optimum amount.